Elastic element for the use in a stabilization device for bones and vertebrae and method for the manufacture of such elastic element

ABSTRACT

A stabilization device for bones or vertebrae comprises a substantially cylindrical elastic element. The elastic element has a first end and a second end opposite to the first end. An elastic section extends between the first end and the second end. The elastic section includes at least first and second helical coils. The first and second helical coils are arranged coaxially so that the first helical coil extends at least in a portion between the second helical coil. The elastic element may form, for example, a portion of a rod, bone anchoring element, or plate.

REFERENCE TO EARLIER FILED APPLICATIONS

The present invention claims the benefit of the filing date under 35U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No.60/563,241, filed Apr. 16, 2004, which is hereby incorporated byreference. The present application also claims foreign priority benefitspursuant to 35 U.S.C. §119(a-d) for German Patent Application Number 102004 018 621.9, filed Apr. 16, 2004 in Germany.

BACKGROUND

The present invention relates to an elastic element for use in a boneanchoring element, a connecting element, a rod, and a stabilizationdevice and a method for manufacturing the same.

It is known to use fixation and stabilization devices to fix fracturesand stabilize spinal columns. These fixation and stabilization devicescommonly comprise at least two bone anchoring elements or bone screws.Each of the bone anchoring elements is anchored in a bone or vertebraand is connected by a rigid plate or a rod. These types of fixation andstabilization devices generally do not allow any movement of the bonesor vertebrae relative to each other.

In some instances, however, it is desirable to stabilize the bones orvertebrae so that the bones or vertebrae can carry out limited,controlled motion relative to each other. This is known as dynamicstabilization. Dynamic stabilization devices commonly comprise anelastic element instead of a rigid plate or rod that connects each ofthe bone anchoring elements.

One example of a dynamic stabilization device for vertebra is disclosedin United States Patent Application Publication No. 2003/0109880 A1. Thedynamic stabilization device comprises first and second screws that areeach anchored in a vertebra. Each of the screws has a receiving memberfor insertion of a spring which thereby connects the screws. The springis provided in the form of a helical spring having closely neighboringcoils like a tension spring. The spring is fixed in the receivingmembers by clamping screws. In this arrangement, however, because thespring is flexible, the spring can evade the pressure of the clampingscrew and therefore become unfixed from the bone screw. Furthermore,both the elasticity and the flexural strength of the spring depend onthe length of the spring. Thus, in applications requiring a spring witha short length, the elasticity and flexural strength of the spring isrelatively small.

Another example of a dynamic stabilization device for a joint such as awrist or knee joint is disclosed in U.S. Pat. No. 6,162,223. The dynamicstabilization device comprises a rod having a proximal rod section and adistal rod section connected to bone pins. The proximal rod section andthe distal rod section are connected to each other by a flexible spring.The proximal rod section, the distal rod section, and the flexiblespring are arranged outside of the body. The proximal rod section andthe distal rod section are not fixedly connected to the flexible spring,but can move freely along a bore therein. In this arrangement, theflexible spring must be formed to have a diameter larger than a diameterof the rod. Additionally, the flexible spring must be large in order tohave a high flexural strength. This dynamic stabilization devicetherefore has a complicated and voluminous structure, which prevents thedynamic stabilization device from being used inside the body on spinalcolumns.

BRIEF SUMMARY

The invention relates to an elastic element for use in a stabilizationdevice for bones or vertebrae. The elastic element comprises asubstantially cylindrical member having a first end, a second endopposite to the first end, and an elastic section between the first endand the second end. The elastic section includes at least first andsecond helical coils. The first and second helical coils are arrangedcoaxially so that the first helical coil extends at least in a portionbetween the second helical coil.

The invention further relates to a stabilization device for bones orvertebrae comprising a substantially cylindrical elastic element. Theelastic element has a first end and a second end opposite to the firstend. At least one of the first and second ends has threads. An elasticsection extends between the first end and the second end. The elasticsection includes at least first and second helical coils. The first andsecond helical coils are arranged coaxially so that the first helicalcoil extends at least in a portion between the second helical coil.

The invention still further relates to a method of manufacturing anelastic element for a stabilization device for bones or vertebrae. Themethod includes providing a substantially cylindrical body and formingfirst and second helical recess in the cylindrical body from an outsideso that the first helical recesses are formed at least in a portionbetween the second helical recesses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of an elastic element according to a firstembodiment;

FIG. 2 a is an elevational view of a double helical spring of theelastic element of FIG. 1;

FIG. 2 b is an exploded view of the double helical spring of the elasticelement of FIG. 1;

FIG. 3 is an elevational view of an elastic element according to asecond embodiment;

FIG. 4 a is an elevational view of an elastic element according to athird embodiment;

FIG. 4 b is a partial exploded view of the elastic element of FIG. 4 a;

FIG. 5 a is an elastic element with a double helical coil sectionaccording to a fourth embodiment of the invention;

FIG. 5 b is a sectional view of the elastic element of FIG. 5 a;

FIG. 6 is a sectional view of an elastic element having a double helicalcoil section according to a fifth embodiment of the invention;

FIG. 7 a is an elastic element having a double helical coil sectionaccording to a sixth embodiment of the invention;

FIG. 7 b is a sectional view of the elastic element of FIG. 7 a

FIG. 8 a is a elevational view of a rod comprising the elastic elementof FIG. 1;

FIG. 8 b is a partial sectional exploded view of a polyaxial bone screwcomprising the elastic element of FIG. 1;

FIG. 8 c is a partial sectional view of a monoaxial screw comprising theelastic element of FIG. 1;

FIG. 8 d is a plan view of a connecting element comprising the elasticelement of FIG. 1;

FIG. 8 e is a sectional view taken along line A-A of FIG. 5 d;

FIG. 9 is a partial sectional view of a stabilization device comprisingseveral of the elastic elements of FIG. 1;

FIG. 10 a is a schematic illustration of a method of manufacturing theelastic element of FIG. 1;

FIG. 10 b is a schematic illustration of a method of manufacturing theelastic element of FIG. 1;

FIG. 10 c is a schematic illustration of a method of manufacturing theelastic element of FIG. 1; and

FIG. 11 is a schematic illustration of a method of manufacturing theelastic element of FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

Various embodiments of the invention are illustrated in FIGS. 1-11 anddescribed herein. Elements of the various embodiments that aresubstantially identical will be referred to with the reference numerals.

FIGS. 1-2 b show an elastic element 1 according to a first embodiment ofthe invention. The elastic element 1 may be made, for example, from abio-compatible material, such as titanium. Examples of otherbio-compatible materials include stainless steel, titanium alloys,nickel-titanium alloys, nitinol, chrome alloy, cobalt chrome alloys,shape memory alloys, materials with super elastic properties, carbonreinforced composites, silicone, polyurethane, polyester, polyether,polyalkene, polyethylene, polyamide, poly(vinyl)fluoride,polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE) and shapememory materials or alloys, such as nickel titanium or nitinol. As shownin FIG. 1, the elastic element 1 is a substantially hollow Cylindricalmember with an outer wall and a continuous coaxial bore 2. The coaxialbore 2 extends from a first end 9 to a second end 9′ of the elasticelement 1 and has a diameter D1. A first helical recess 3 is formed inthe outer wall in a direction of a central axis M of the cylindricalmember. The first helical recess 3 has a height H and opens into thecoaxial bore 2 in a radial direction. The first helical recess 3 isformed at a predetermined angle a and extends over a predeterminedlength L of the outer wall. A second helical recess 4 is formed in theouter wall in-between the first helical recess 3 in the direction of thecentral axis M of the cylindrical member. The second helical recess 4 isformed at substantially the same angle a and extends over substantiallythe same length L of the outer wall as the first helical recess 3. Thesecond helical recess 4 opens into the coaxial bore 2 in the radialdirection.

First and second internal threads 5, 5′ are formed at the first andsecond ends 9, 9′, respectively, of the elastic element 1. The first andsecond internal threads 5, 5′ extend over a predetermined length in anaxial direction. The lira and second internal threads 5, 5′ do notoverlap or extend into the first and second helical recesses 3, 4 formedin the outer wall. The elastic element 1 has an outer diameter, which isselected according to the desired use thereof. The length L of the firstand second helical recesses 3, 4 in the direction of the central axis Mof the cylindrical member, the height H of the first and second recesses3, 4, the angle a of the helices along which the first and secondhelical recesses 3, 4 are formed, and the diameter D1 of the coaxialbore 2 is selected to provide a desired stiffness to the elastic element1 with respect to axial forces F_(ax), bending forces F_(B) andtorsional forces F_(T) acting on the elastic element 1.

As shown in FIGS. 2 a-2 b, a double helical spring or elastic section 6consisting of a first helical coil 7 and a second helical coil 8 isformed by the first and second helical recesses 3, 4. Coils of the firsthelical coil 7 extend between coils of the second helical coil 8. Thefirst and second helical coils 7, 8 are substantially identical and havesubstantially the same angle a. The coils of the first helical coil 7are rotated approximately 180 degrees with respect to the coils of thesecond helical coil 8 around the central axis M, which is common to boththe first and second helical coils 6, 7, so that the first and secondhelical recesses 3, 4 oppose each other. The coils of the first helicalcoil 7 therefore run midway between the coils of the second helical coil8 and vice versa. It will be appreciated by those skilled in the artthat the elastic member 1 may additionally comprise more than two of thehelical coils, wherein coils of each of the helical coils extendin-between coils of adjacent helical coils.

In order to obtain optimal elastic properties in an elastic element (notshown) with a single helical spring (not shown) having a predeterminedlength, the angle of the helices of the single helical spring (notshown) must be formed to have at least one whole turn. In the doublehelical spring 6 shown in FIGS. 2 a-2 b, however, the first helical coil7 and the second helical coil 8 require less than one whole turn toobtain optimal elastic properties even though the double helical spring6 has the same predetermined length as the single helical spring (notshown). Unlike the angle of the helices of the single helical spring(not shown), the angle a of the helices of the double helical spring 6may therefore be increased to increase the flexural strength of theelastic element 1. Additionally, the elastic element 1 may be formed,for example, to have an ovular cross-section or to be waisted such thatthe elastic element 1 has a flexural strength which is dependent ondirection. The elastic element 1 therefore has a high flexural strengthand a short length such that the elastic element 1 may be handled easilywhile at the same time providing a high operational reliability.Additionally, the elastic element 1 may be combined with other elementsin various different ways to be a dynamic stabilization device forvertebrae or bones.

FIG. 3 shows an elastic element 11 according to a second embodiment ofthe invention. The elastic element 11 is a substantially hollowcylindrical member having first and second helical recesses 13, 14formed in an outer wall thereof to form a double helical spring orelastic section. The double helical spring is formed in a similarfashion to the first embodiment. The elastic element 11 of the secondembodiment differs from the first embodiment in that the elastic element11 has a coaxial bore 12 that extends partially through the cylindricalmember. The coaxial bore 12 extends from a first end 17 over the lengthL of the double helical spring and is coaxial with the central axis M ofthe cylindrical member. Internal threads 15 are provided in the coaxialbore 12 adjacent to the first end 17. At a second end 17′, which opposesthe first end 17, the elastic element 11 is provided with a cylindricalprojection 16. The cylindrical projection 16 has external threads.Alternatively, the coaxial bore 12 may have a diameter smaller than anouter diameter of the cylindrical projection 16 and may extend throughthe entire cylindrical member.

FIGS. 4 a-4 b show an elastic element 21 according to a third embodimentof the invention. The elastic element 21 is a substantially cylindricalmember having first and second helical recesses 24, 25 formed in anouter wall thereof to form a double helical spring or elastic sectionconsisting of a first helical coil 26 and a second helical coil 27. Thedouble helical spring is formed similar to the first and secondembodiments. The elastic element 21 of the third embodiment differs fromthe first and second embodiments in that the elastic element 21 does nothave a bore coaxial with the central axis M of the cylindrical member.The elastic member 21 has a first end 22 and a second end 22′. The firstand second ends 22, 22′ have first and second cylindrical projections23, 23′, respectively. The first and second cylindrical projections 23,23′ have external threads.

An elastic element according to a fourth embodiment is shown in FIG. 5a. FIG. 5 b is a sectional view of the elastic element of FIG. 5 a. Theelastic element 30 according to the fourth embodiment differs from theelastic element according to the first embodiment in that the pitch a ofthe recesses 31, 32 which form the double helical coil is not constantbut varies over the length L of the double helical coil of the elasticelement 30. The pitch a varies in such a way that the distance) L of therecesses 31, 32 increases from the free ends of the elastic element 30towards the middle. Accordingly, the bending stiffness of the elasticelement 30 varies and increases with increasing distance L of therecesses 31, 32. By varying the pitch of the recesses along the centralaxis of the elastic element, it is possible to achieve a particularstiffness at a particular position. Similar to the elastic element ofthe first embodiment, the elastic element 30 according to the fourthembodiment has coaxial bore 34 having an inner diameter D1 and innerthreads 33, 33′ extending a predetermined length from the free end,respectively. FIG. 6 shows sectional view of an elastic elementaccording to a fifth embodiment.

The elastic element according to the fifth embodiment differs from theelastic element 30 according to the fourth embodiment in that the innerdiameter D1 of the continuous coaxial bore 42 is not constant but variesof the length L′ of the elastic element 40. The inner diameter D1 of thebore 42 varies in such away that it decreases from the free ends towardsthe middle of the elastic element 40. Accordingly, the final stiffnessof the elastic element 40 varies and increases with decreasing innerdiameter D1. By varying the inner diameter of the coaxial bore, thestiffness of the elastic element 40 can be varied at differentpositions.

Similar to the fourth embodiment the elastic element 40 includes asection with an inner thread 41, 41′ having a predetermined lengthadjacent to each of its free end, respectively.

In FIG. 7 a an elastic element according to a sixth embodiment is shown.FIG. 7 b is a sectional view of the elastic element of FIG. 7 a.

The elastic element 35 according to the sixth embodiment differs fromthat of the fifth embodiment in that the outer diameter D2 of theelastic element 35 is not constant but varies over the length L′ of theelastic element 35. The outer diameter D2 varies in such a way that itincreases from the free ends towards the middle of the elastic element35. Accordingly, the bending stiffness of the elastic element 35 variesand increases with increasing outer diameter. Therefore, a position witha desired bending stiffness can be obtained by adjusting the outerdiameter of the elastic element.

Similar to the fourth embodiment the elastic element 35 has adjacent toits free ends a section with an inner thread 36, 36′ of a predeterminedlength, respectively, and a continuous coaxial bore 37 with an innerdiameter D1. Recesses 38 and 39 to form the double helical coil areformed like in the other embodiments.

In the fourth to sixth embodiments of the instant invention, the bendingstiffness of the elastic element increases from the free ends towardsthe middle of the elastic element. However, by appropriate selection ofthe pitch a of the recesses, the outer diameter D2 of the elasticelement and the inner diameter D1 of the coaxial bore, the bendingstiffness can be adjusted to have a desired stiffness at a particularposition along the length L, L′ of the double helical coil of theelastic element.

FIG. 8 a illustrates a first example of an application of the elasticelement 1. As shown in FIG. 8 a, the elastic element 1 may form aportion of a rod 50, which may be used, for example, to connect pediclescrews (not shown) at a spinal column (not shown). The rod 50 in theillustrated embodiment consists of the elastic element 1 and first andsecond end portions 51, 51′. The first and second end portions 51, 51′each have a cylindrical projection (not shown) with external threads(not shown) that cooperates with the first and second internal threads5, 5′, respectively, of the elastic element 1, shown in FIG. 1.Alternatively, an external nut (not shown) or other attachment membermay be used to fix the elastic element 1 to the first and second endportions 51, 51′. In the illustrated embodiment, the first and secondend portions 51, 51′ and the elastic element 1 have approximately thesame outer diameter. The first and second end portions 51, 51′ have alength that may be selected independently from the length L of theelastic element 1, which is shown in FIG. 1. The length of the first andsecond end portions 51, 51′ and the length L of the elastic element 1selected depends on a desired end application. Because the rod 50 isformed with the elastic element 1, the rod 50 can absorb compressionforces, extension forces, bending forces and torsional forces to apredetermined extent by means of the elastic properties of the elasticelement 1.

FIG. 8 b illustrates a second example of an application of the elasticelement 1. As shown in FIG. 8 b, the elastic element 1 may form aportion of a bone anchoring element, such as a polyaxial bone screw 60.The polyaxial bone screw 60 in the illustrated embodiment includes ascrew 61 with a shaft 62 and a head 63. The shaft 62 has a tip (notshown) and includes bone threads 64 for screwing into a bone (not shown)and. A cylindrical projection (not shown) extends from the shaft 62 on aside opposite from the tip (not shown) and has external threads (notshown) that cooperate with the internal threads 5 of the elastic element1, which are shown in FIG. 1. As shown in FIG. 8 b, the head 63 has acylindrical section 65 adjacent thereto. A cylindrical projection (notshown) extends from the cylindrical section 65 and has external threads(not shown) that cooperate with the internal threads 5′ of the springelement 1, which are shown in FIG. 1.

As shown in FIG. 8 b, the screw 61 is pivotally held in a receivingmember 66 in an unloaded state. The receiving member 66 is substantiallycylindrical and has a first receiving member bore 67 and a secondreceiving member bore 68. The first receiving member bore 67 is providedat a first end of the receiving member 66. The first receiving memberbore 67 is substantially axially symmetrical and has a diameter largerthan a diameter of the shaft 62 but smaller than a diameter of the head63. The second receiving member bore 68 is substantially coaxial andopens at a second end of the receiving member 66 opposite the first end.The second receiving member bore 68 has a diameter large enough that theshaft 62 of the screw 61 may be guided through the second end and thesecond receiving member bore 68 until the head 63 abuts an edge of thefirst receiving member bore 67. The receiving member 66 has asubstantially U-shaped recess 69, which extends from the second endtowards the first end. The substantially U-shaped recess 69 forms firstand second legs 70, 70′ with free ends. In a region adjacent to the freeends, the first and second legs 70, 70′ have internal threads, whichcooperate with corresponding external threads of a securing element 71that fixes a rod 72 in the receiving member 66.

A pressure element 73 that is provided for fixation of the head 63 inthe receiving member 66 has a concave recess 74 on a side facing thehead 63. The concave recess 74 has a radius substantially identical to aradius of the head 63. The pressure element 73 has an outer diameterselected so that the pressure element 73 can be inserted into thereceiving member 66 and can slide towards the head 63. The pressureelement 73 has a coaxial pressure element bore 75 for providing accessto a tool receiving recess (not shown) in the head 63.

During assembly, the cylindrical projection (not shown) of the shaft 62is screwed into the internal threads 5 of the elastic element 1 and thecylindrical projection (not shown) of the cylindrical section 65 of thehead 63 is screwed into the internal threads 5′ of the elastic element 1to form the screw 61. The shaft 62 of the screw 61 is then inserted intothe second end of the receiving member 66 and guided through the secondreceiving member bore 68 until the head 63 abuts the edge of the firstreceiving member bore 67. The pressure element 73 is inserted into thesecond receiving member bore 68 so that the concave recess 74 ispositioned adjacent to the head 63. The screw 61 is screwed into a bone(not shown) or vertebra (not shown). The rod 72 is inserted into thereceiving member 66 and is arranged between the first and second legs70, 70′. The angular position of the screw 61 relative to the receivingmember 66 is then adjusted and fixed with the securing element 71.

Because the screw 61 is formed with the elastic element 1, the screw 61may be diverted from the angular position by a limited extent.Additionally, if the elastic element 1 protrudes at least partiallyabove a surface of the bone (not shown), the elastic element 1 canabsorb compression forces, extension forces, bending forces andtorsional forces because of the elastic properties of the elasticelement 1. If the elastic element 1 does not at least partially protrudeabove the surface of the bone (not shown), the screw 61 can stillslightly yield, when the bone (not shown) or vertebra (not shown) movessuch that the occurrence of unfavorable stress is avoided.

FIG. 8 c illustrates a third example of an application of the elasticelement 1. As shown in FIG. 8 c, the elastic element 1 may form aportion of a bone anchoring element, such as a monoaxial screw 80. Themonoaxial screw 80 in the illustrated embodiment consists of a headformed as a receiving member 81 and a shaft 86. The receiving member 81has a substantially U-shaped recess 83 formed at a first end thereof.First and second legs 84, 84′ are formed by the U-shaped recess 83. Thefirst and second legs 84, 84′ receive a rod 82 therebetween. Internalthreads (not shown) that correspond to external threads on securingmember 85 are formed on inside surfaces of the first and second legs 84,84′. The rod 82 is clamped between a bottom surface of the U-shapedrecess 83 and the securing member 85 when the securing member 85 isengaged with the internal threads (not shown). A cylindrical projection(not shown) extends from a second end of the receiving member 81opposite from the first end. The cylindrical projection (not shown) hasexternal threads (not shown) that correspond to the internal threads 5′of the elastic element 1, which are shown in FIG. 1. The shaft 86 issimilar to the shaft 62 previously described and has a cylindricalprojection (not shown) extending therefrom with external threads (notshown) that corresponds to the internal threads 5 of the elastic element1, which are shown in FIG. 1.

During assembly, the cylindrical projection (not shown) of the shaft 86is screwed into the internal threads 5 of the elastic element 1 and thecylindrical projection (not shown) of the receiving member 81 is screwedinto the internal threads 5′ of the elastic element 1 to form themonoaxial screw 80. The monoaxial screw 80 is screwed into a bone (notshown) or vertebra (not shown). The U-shaped recess 83 is aligned andthe rod 82 is inserted into the receiving member 81 and is arrangedbetween the first and second legs 84, 84″. The rod 82 is then fixed bythe securing member 85.

FIGS. 8 d-8 e illustrate a fourth example of an application of theelastic element 1. As shown in FIG. 8 d, the elastic element 1 may forma portion of a connecting element 90. The connecting element 90 in theillustrated embodiment consists of a rod 91 and a plate 92. The rod 91has a cylindrical,projection (not shown) with external threads thatcorrespond to the internal threads 5 of the elastic element 1, which areshown in FIG. 1. A cylindrical projection (not shown) extends from theplate 92 and has external threads (not shown) corresponding to theinternal threads 5′ of the elastic element 1, which are shown in FIG. 1.As shown in FIG. 8 d, the plate 92 has a first section 93 and a secondsection 93′ connected by a bridge 94. The first and second sections 93,93′ are substantially circular from a top view. The bridge 94 has awidth B smaller than a diameter D of the first and second sections 93,93′. The first and second sections 93, 93′ each have a screw receivingbore 95, 95′, respectively, formed coaxially with the first and secondsections 93, 93′. The screw receiving bores 95, 95′ have a shape adaptedfor the reception of countersunk screws (not shown). As shown in FIG. 8e, a first side 96 of the plate 92 has a convex curvature and a secondside 97 of the plate 92 has a concave curvature for abutting a surfaceof a bone (not shown). Due to the different curvatures of the first andsecond sides 96, 97, the plate 92 tapers towards lateral edges 98, 98′.The plate 92 is, therefore, stable and compact.

Modifications of the rod 50, the polyaxial bone screw 60, the monoaxialscrew 80, and the connecting element 90, shown in FIGS. 8 a-8 e are alsopossible. For example, the elastic element 1 in the rod 50, thepolyaxial bone screw 60, the monoaxial screw 80, and the connectingelement 90 is illustrated as being a separate element that requiresconnection therewith. Alternatively, the elastic element 1 may beintegrally formed with the polyaxial bone screw 60, the monoaxial screw80, and the connecting element 90 or press-fit thereto.

FIG. 9 illustrates a fifth example of an application of the elasticelement 1. As shown in FIG. 9, the elastic element 1 may form a portionof a stabilization device 100 that is used, for example, in spinalcolumns. The stabilization device 100 in the illustrated embodimentconsists of first and second bone anchoring elements 101, 101′,respectively, connected by a rod 103. Each of the first and second boneanchoring elements 101, 101′ has a screw 102, 102′, respectively, formedwith an elastic element 1. The rod 103 is also formed with an elasticelement 1. Each of the screws 102, 102′ is screwed into a vertebra 104,104′ so that a dynamic stabilization is established between thevertebrae 104, 104′ and the stabilization device 100. Because the rod103 and the screws 102, 102′ are made of several elements, thestabilization device 100 has various properties by the combination ofonly a few basic elements. The stabilization device 100 is not limitedto the embodiment illustrated and depending on a desired applicationthereof, it is possible, for example, to provide only the rod 103 withthe elastic element 1.

A method of manufacturing the elastic element 1 by wire electricaldischarge machining (EDM) is shown in FIGS. 10 a-10 c. As shown in FIG.10 a, a first bore 110 is formed in a solid cylinder 112 of abiocompatible material, such as titanium, perpendicular to a centralaxis M′ of the cylinder 112. The first bore 110 extends through thewhole cylinder 112. A second bore 111 is formed coaxial with the centralaxis M′ of the cylinder 112 so that the cylinder 112 is made hollow. Theorder of forming the first and second bores 110, 111 is arbitrary andmay be varied according to a desired manufacturing process. A wire 113for wire EDM is guided through the first bore 110 in a directionindicated by arrow P.

As shown in FIG. 10 b, wire EDM is performed by moving the cylinder 112in a direction indicated by arrow X along the central axis M′. Thecylinder 112 is moved at a constant feed rate relative to the wire 113and is simultaneously rotated around the central axis M′ in a directionindicated by arrow R with a constant angular velocity. Only relativemovement of the wire 113 relative to the cylinder 112 is relevant.Accordingly, either the wire 113 or the cylinder 112 may be fixed duringthe wire EDM. As the cylinder 112 is rotated, first and second helicalrecesses 114, 115 are formed.

As shown in FIG. 10 c, after the first and second helical recesses 114,115 have been formed over a predetermined length of the cylinder 112along the central axis M′, the rotation of the cylinder 112 is stopped.FIG. 10 c shows the elastic element 1 shortly before completion of thewire EDM. The wire EDM thereby simultaneously forms in the outer wall ofthe cylinder 112, first and second helical recesses 114, 115 havingapproximately identical angles, which open in a radial direction intothe second bore 111.

As shown in FIG. 11, a first run-out 120 may be formed at a beginning ofthe wire EDM and at a second run-out 120′ may be formed at an end of thewire EDM. The first and second run-outs 120 and 120′ have aconfiguration by which load peaks can be minimized in the material at atransition from the elastic section to the rigid section duringoperation. The first and second run-outs 120, 120′ may have, forexample, a semi-circular configuration. The first and second run-outs120, 120′ advantageously may be made in one common manufacturing step.Additionally, unlike during the manufacture of a single helical spring(not shown), during the manufacture of the elastic element 1, switchingbetween each axis of the wire EDM machine is not necessary. Internalthreads are then formed along the central axis M′ in end sections of thesecond bore 111 adjacent to the first and second ends.

Alternatively, the elastic element 1 may be milled. A first helicalrecess is milled along a first helix of a central axis of a solidcylinder formed of a bio-compatible material, such as titanium, having apredetermined outer diameter. The first helical recess is formedcollinear with the central axis of the cylinder by a side mill. A secondhelical recess is milled along a second helix of the central axis suchthat coils of the second helical recess run between coils of the firsthelix. A bore is formed along the central axis of the cylinder over thewhole length of the cylinder so that the first and second helicalrecesses communicate with the bore. The first and second helicalrecesses have first and second run-outs, respectively. The first andsecond run-outs of the first and second helical recesses at a transitionbetween the first and second helices and end sections of the cylinderhave a large influence on the stability of the elastic element 1. Thefirst and second run-outs of the first and second helixes at both of theend sections are reworked by an end mill so that Sharp edges on aninternal surface of the bore are removed. The first and second run-outsare milled by the end mill at an angle that is tangential relative to ahelical line. The part is then chamfered on an inside and on an outside.Internal threads are then formed along the central axis in the endsections of the bore adjacent to first and second ends of the cylinder.

Further alternative methods for manufacturing the elastic element 1 are,for example, laser milling or hydro milling. These methods are performedsimilar to the wire EDM method, but instead of simultaneously formingthe first and second helical recesses by a wire, a laser beam or a waterbeam is used. Additionally, instead of forming at least one of theinternal threads, a cylindrical projection with external threads may beformed at a beginning of any one of the manufacturing methods by alathe. In this instance, the bore has a diameter smaller than a diameterof the cylindrical projection. The spring element 1 may also be formedwithout the bore.

The embodiments described above and shown herein are illustrative andnot restrictive. The scope of the invention is indicated by the claims,including all equivalents, rather than by the foregoing description andattached drawings. The invention may be embodied in other specific formswithout departing from the spirit and scope of the invention.

1-25. (canceled)
 26. A bone anchoring element for bones or vertebrae,comprising: a shaft with a free end; a receiving portion opposite to thefree end along a longitudinal axis of the shaft, the receiving portionhaving a U-shaped recess opening in a direction away from the free endof the shaft and forming two legs for accommodation of a rodtherebetween; and a locking element configured to secure to thereceiving portion to lock the rod in the U-shaped recess; wherein afirst section of the shaft has a first end and a second end spaced apartalong the longitudinal axis, the first section being cylindrical andhaving an exterior surface that is entirely threadless from the firstend to the second end and an interior surface defining a coaxial boreextending from the first end of the first section to a portion of theshaft between the second end of the first section and the free end ofthe shaft; and wherein the first section of the shaft further defines atleast two recesses each extending through a wall of the first sectionand forming an opening on the exterior surface of the first section,wherein the exterior surface of the first section completely separatesthe respective openings of the at least two recesses from one anotheralong an entire length of the cylindrical section from the first end tothe second end.
 27. The bone anchoring element of claim 26, wherein theshaft further comprises a second section having an exterior surface thatis threaded.
 28. The bone anchoring element of claim 26, wherein thefirst section of the shaft comprises first and second helical coilsegments arranged coaxially around the coaxial bore and separated fromone another in a circumferential direction by the at least two recesses.29. The bone anchoring element of claim 28, wherein the first and secondhelical coils are substantially identical to one another.
 30. The boneanchoring element of claim 28, wherein each of the first and secondhelical coils extends along an entire length of the cylindrical sectionfrom the first end to the second end.
 31. The bone anchoring element ofclaim 28, wherein the first helical coil is rotated approximately 180degrees with respect to the second helical coil.
 32. The bone anchoringelement of claim 28, wherein the first and second helical coils each hasa pitch, and wherein the pitches of the first and second helical coilsare identical.
 33. The bone anchoring element of claim 26, wherein thefirst section of the shaft is elastic.
 34. The bone anchoring element ofclaim 26, wherein the bone anchoring element comprises a bio-compatiblematerial.
 35. The bone anchoring element of claim 26, wherein at leastone of the first end and the second end of the first section of theshaft is provided with internal threads.
 36. The bone anchoring elementof claim 26, wherein at least one of the first end and the second end ofthe first section of the shaft is provided with a cylindrical projectionhaving an external thread.
 37. The bone anchoring element of claim 26,wherein the each of the recesses extends in a radial direction relativeto the longitudinal axis through the wall of the first section of theshaft.
 38. The bone anchoring element of claim 26, wherein the shaft andthe head are monoaxially connected.
 39. The bone anchoring element ofclaim 26, wherein the shaft has a spherical segment-shaped head at anend of the shaft opposite the free end, and wherein the receivingportion is a separate part which is provided with a seat for pivotablyholding the head.
 40. The bone anchoring element of claim 39, furthercomprising a pressure element configured to lock the head against theseat.
 41. The bone anchoring element of claim 26, wherein the coaxialbore extends partially through the shaft and ends at the portion of theshaft between the second end of the first section and the free end ofthe shaft.